Chip-to-chip communication is a central aspect of electronic devices. An example is a processor that is connected to a memory interface where communication takes place between chips located in the same device. Another example is a television that is connected to the digital output of a set-top box. In this case, the two chips communicating are located in different devices. A third example is system-on-chip communications, where multiple chips are integrated into a single package and communication takes place between the chips in the same package.
Communication between these chips can take place over several different transmission media. A single unit of such a transmission medium able to carry information is referred to herein as a “wire” but this is to be understood generally. Chips contained within a single device might be mounted on a printed circuit board (PCB) or packaged together. In the first case, the wires usually take the form of microstrip or stripline traces on a PCB. In the latter case, there are several options to connect the chips. For chips located in two different devices, the wires can be physical copper wires or optical fibers. Often multiple wires are bundled into a communication bus to increase the total bandwidth.
In such chip-to-chip communication systems, the particular configuration can really affect power consumption, speed of communication, error-performance, bus width and noise resilience. There are several important trade-offs. Increasing the power consumption of the bus can lead to better error performance and noise resilience. However, low power consumption is preferred in most electronic devices, more so when the device runs on a battery. By using a larger bus width, the total transfer rate can be increased. However, a larger bus width may require the chip to contain more pins, which can be a problem since pins are often a scarce resource.
To signal over a bus, a signaling method is used. This signaling method generates a signal for each wire of the bus based on digital information. A commonly used signaling method is single-ended signaling. Single-ended signaling is not very resilient against noise and half of the transmission power is wasted in a DC component of the transmitted signal. An alternative to single-ended signaling is differential signaling. Differential signaling provides good noise resilience and is more efficient in power compared to single-ended signaling. Examples of systems that employ differential signaling are RS-422, RS-485, twisted-pair Ethernet, DVI and HDMI. The downside of differential signaling is that it conventionally requires twice as many wires than signals that are transmitted on the bus.
In chip-to-chip communications, there are several sources of noise, such as: (a) common-mode noise (noise and interference that is common to the whole bus); (b) independent noise that is added to each of the wires of the bus individually; and (c) simultaneous switching output (“SSO”) noise, which is caused by variations in the current through the circuits driving the wires of the bus.
Single-ended signaling is sensitive to common-mode noise and introduces SSO noise. Differential signaling is a good alternative that does not suffer from these two issues. Furthermore, differential signaling is more efficient in terms of the power required to achieve a given error performance. However, differential signaling conventionally requires twice the number of wires than bits transmitted on the bits per time interval. Furthermore, a substantial amount of transmission power is required to assure a good error and noise performance.
Orthogonal Differential Vector Signaling (“ODVS”), as disclosed CS-1 for example, describes a method that has similar noise performance as differential signaling and can provide pin efficiencies close to single-ended signaling. In some embodiments described in CS-1, this is achieved by transforming the incoming signals using an orthogonal or unitary transformation.
In many applications, it is preferable to increase the noise resilience of a communications system, as described above, at the expense of the pin-efficiency. Where differential signaling is used, pin-efficiency is sacrificed to obtain a high resilience to certain types of errors. Where ODVS is used, the pin-efficiency is increased while maintaining the resilience of the transmission against similar amounts of noise as in differential signaling.
However, sometimes even better performance is needed for chip-to-chip signaling where power consumption is limited and/or pin availability is limited.